1). Field of the Invention
This invention relates to a method of making a semiconductor transistor.
2). Discussion of Related Art
Integrated circuits are usually manufactured in and on silicon and other semiconductor substrates. An integrated circuit may include millions of interconnected transistors that are formed over an area of a few square centimeters.
Such a transistor usually includes a gate dielectric layer on the silicon substrate, a gate electrode on the gate dielectric layer, and source and drain regions in the silicon substrate on opposite sides of the gate electrode. The source and drain regions are usually made by implanting dopant impurities into the silicon substrate and subsequently heating or “annealing” the entire structure to cause diffusion of the dopant impurities into the silicon substrate. No barrier exists in the silicon substrate that would limit diffusion of the dopant impurities, so that the locations that the dopant impurities diffuse to cannot be tightly controlled.
Achieving high transistor performance requires very high implant doses and tight control over dopant placement and diffusion. One option might be to form source and drain recesses on opposite sides of the gate electrode, and subsequently filling the recesses with doped semiconductor material. Dopant impurities such as boron may, for example, be deposited together with a semiconductor material such as germanium. An additional advantage of doped germanium is that it has a lower sheet resistance than doped silicon. Such insitu doping of germanium with boron is, however, difficult to achieve.